This research will involve laboratory studies of the development of aggression and predation in the grasshopper mouse, Onychomys torridus. This species is a carnivorous rodent showing highly developed prey- killing behavior and pronounced aggressive behavior toward individuals of its own species and other rodent species. It affords an ideal opportunity for descriptive and experimental analysis of the ontogeny of these behaviors. The particular focus of this study will be to analyze parental influences on the ontogeny of predation and aggression by means of cross-fostering Onychomys and Peromyscus. The latter is a non- predatory, less aggressive genus with a number of biological similarities. Cross-fostering will be used as one of several experimental techniques to study the role of early experiences and social influences in the development of various types of aggression and predation in this species. Studies will also be directed toward measurements of the relative stress responses to various types of aggressive and predatory episodes.